Putting aside the obvious call for nostalgia.
Elder Scrolls expansions typically add some sort of land to the world. So far we've seen an island landmass, isolated cities, and a deadric realm. It makes sense that any expansion to Skyrim would follow suit and add some size to the world itself rather than simply taking place within the world. And before it comes up, I don't view KotN as an expansion. If we accept KotN as an expansion, content wise, we may be in for some serious nickel and dime-ing in the future.
Anyway, I think Bethesda is a little weary of throwing yet another daedric realm at us again. Last game we saw multiple oblivion worlds, even if they all looked like the Deadlands, barring SI of course. So this leaves us with some sort of distant city type of area, or full-on additional landmass. Something mundane. Something on Nirn.
- Orsinium/Eastern High Rock - Orsinium sits north of Wayrest, smack dab in the middle of High Rock with previously unexplored parts of High Rock between it and Skyrim. This could be used in the future should High Rock ever be revisited and given a full province wide game ala Oblivion and Skyrim.
- Hammerfell - Unexplored for the most part. I would rather see a full TES game dedicated to Hammerfell than see another chunk of it in expansion form.
Cyrodiil - I highly doubt Bethesda would go back to Cyrodiil in an expansion so soon. Maybe if they simply did the Imperial City properly (enormous) the IC could have an expansion in and of itself like Mournhold, but I still don't see this happening.
- Northwestern Morrowind - Seeing as it's likely still in ruins, I'm not sure how much they could do with the place. I was kind of hoping to see Morrowind in full at some point, but after it got blowed up, I'm not sure I ever will
- Valenwood/Elsweyr/Black Marsh/Summerset Isle - Too far from Skyrim to make the cut, and all of them are prime candidates for future full TES games.
I'm also not big on the idea of having only chunks of provinces to explore. It worked in Daggerfall given the gargantuan size of the world back then. Having arbitrary cutoff points is hard to justify, unlike the clearly defined borders of a full province.
- Roscrea - Probably stands the biggest chance of getting picked for any expansions. It's near (ish) Skyrim, is not well explored, and has a link to Solitude and so may be contested territory between Nordic city-states.
- Solstheim - Second most likely place for an expansion IMO. It's been explored in the past, but that was before the Red Year and subsequent invasion. Now populated by Dunmer refugees, the land may have been developed extensively. Since the land has already been mapped in Bloodmoon, only new Dunmeri cities would need to be placed which would allow Bethesda to work out the landmass rather quickly, allowing them to concentrate more heavily on quests, dungeons, creatures, and the lore surrounding what is essentially the second Dunmer/Chimer exodus.
If Bethesda does not feel the need to visit another Skyrim-esque landscape, the eruption of Red Mountain may well have altered the climate of Solstheim to be somewhat more like Morrowind used to be.
- Cathnoquey/Esroniet/Yneslea - The lands sitting between Tamriel and Akavir. Not well explored, but far removed from Skyrim. What our protagonist would be doing there would be hard to place.
- Atmora/Akavir/Pyandonea/Thras/Yokuda - Mysterious lands that I feel should never be explored too thoroughly. The mystery surrounding them is part of what gives them charm.
- Aetherius - This is stretching things a bit. The hero of Daggerfall was there once, but I do not imagine the aedra still living there would take kindly to intruders and so would likely be quite desolate.
- A new landmass - Much like Solstheim was to Morrowind, and new landmass could be created and given history specifically for Skyrim. Always a possibility.
In all honesty, I can't see an expansion taking place outside of Roscrea, Solstheim, or Atmora. Nor do I see Bethesda sending us off to a huge, fenced in, distant city which will likely be pruned in the future to accommodate a full TES game. Making our return very underwhelming. Of those three, Roscrea and Solstheim have the best chances of making it in. Roscrea being mostly untouched to this point, can be developed however Bethesda sees fit. Solstheim is familiar territory that has recently been involved in a good number of lore happenings and could be quickly mapped out allowing Bethesda to concentrate on more important things. Do note that, depending on the technology available, it may actually be harder to replicate Solstheim than to generate Roscrea from scratch. Heightmap wise anyway.
So yeah, there's my take on the feasibility of various locations for possible expansions.